Static synchronous compensators (“STATCOMs”) are power electronic converter systems used for controlling reactive current flow to/from an electric power system. A typical STATCOM is made from a three-phase voltage source inverter with all three legs (also referred to herein as poles or phases) of the inverter connected to the same DC bus. This allows transient power flowing into/out of a given phase to be cancelled out by the power flow out of/into the other two phases, which occurs naturally in STATCOM applications. As a result, the DC bus voltage does not significantly deviate during normal STATCOM operation. The maximum value of the DC bus voltage is limited by the voltage rating of the devices comprising the voltage source inverter. This maximum DC bus voltage then imposes a limit on the AC voltage that can appear between phase legs. Unfortunately, this AC voltage is often too low to directly connect to an electric power system so the use of a step-up transformer is often required to enable operation at higher voltages.
Another type of STATCOM uses a multi-level converter called a cascaded H-Bridge converter (CHB) which enables operation at higher voltages and often eliminates the need for a step-up transformer. This is beneficial since the step-up transformer can be inefficient and lossy. In contrast to the three-phase converter STATCOM described above, CHB STATCOMs are natively single phase; the H-Bridge converters are not connected to the same DC bus. When identical voltages, Vdc, are used for each capacitor of the cascaded converter, the total converter voltage of a given pole can take any integer multiple of Vdc between −Ncells×Vdc and +Ncells×Vdc, where Ncells is the number of CHB H-Bridge cells. The total converter voltage is used to control the pole current, which flows through all individual H-Bridge cells in a given leg, since they are connected in series.
Another advantage of CHB STATCOMs is that they more easily create a high fidelity (low harmonic) AC voltage waveform from the lower voltage rated cells in series by virtue of their smaller voltage step size. In addition, the converter can continue to operate even with a failed cell through bypassing (shorting out) the level with the failed cell.
The CHB STATCOMs are switched or modulated according to various switching/modulation schemes to produce a desired output waveform. Typical modulation schemes include staircase modulation, phase shifted modulation, and level shifted modulation. Each of these modulation schemes has benefits as well as disadvantages in relation to four key performance criteria. These criteria are: 1) minimizing the number of switching events to reduce power loss, (2) balancing the isolated capacitor voltages of the CHB cells to keep each cell voltage within safe operating levels, (3) producing a high fidelity AC voltage waveform to minimize the passive filter components on the AC side of the converter, and (4) allowing for even distribution of losses among the CHB cells to prevent accelerated wear out of an individual cell.
With prior art switching schemes, these four objectives are inherently at odds with each other, i.e. improving one or two of these objectives usually comes at the expense of the others. With the staircase switching scheme there are low power losses due to a minimum number of switching events, but there are significant drawbacks, such as poor cell voltage balancing, a low fidelity waveform, and uneven distribution of losses among cells. The phase shifted carrier modulation scheme performs well in most areas, except for power loss, in which case it performs poorly. It is critical to minimize power losses for multiple reasons, including: to minimize STATCOM cost and physical footprint; to maintain low component temperatures; and to maximize component lifetimes. The level shifted carrier modulation scheme performs well in all areas except for the balancing of the capacitor voltages. This is a major detriment for STATCOM applications, since the DC buses are not supplied or balanced by an external power source.